Mutations in the recF gene causes a reduction in the expression of the SOS response. Increasing the levels of recA protein in recF mutants, either by introducing a multicopy plasmid bearing the recF gene or a recA operator constitutive mutation, has no effect on the recF phenotype. However, introduction of a recA mutation (tif-1) which produces a recA protein which, upon incubation at 42 degrees C, expresses the recA proteolytic activity required for expression of the SOS response suppresses the UV sensitivity due to recF. This suggests that recF may be required for the activation of the protease activity of the recA protein. We are now investigating this possibility.